Silicon resins are typical coating agents that make a hydrophilic surface over glass lenses, bathroom mirrors, injection needles, etc. to produce antifogging effects or the like. The silicon resins mixed with photocatalytic anatase TiO2 are also well known in the art as the coating agents which can form a hydrophilic film on a surface of base material and produce hydrophilic effects upon exposure to ultraviolet rays.
The hydrophilic surface films comprising the silicon resins attract dust in the air by static electricity and are blackened as a result. The hydrophilic coating agents comprising the photocatalytic anatase TiO2 and the silicon resin, need ultraviolet rays to produce hydrophilic effects. That is, no hydrophilic effects will be obtained when the agents have been applied to areas where the ultraviolet rays do not reach.
To overcome these problems, Japanese Patent Laid-Open Publication No. 237353/1998 discloses a hydrophilic coating agent that comprises amorphous titanium oxide and silicon oxide. The examples of the silicon oxide include silicon dioxides such as colloidal silica; siloxane compounds such as silicone and organopolysiloxane; and water glass.
However, these are disadvantage in that:
the colloidal silica has a poor dispersibility in combination with a photocatalytic compound (titanium oxide), particularly when the photocatalytic compound is ultrafine powder;
the siloxane compounds have an insufficient dispersibility and a film obtained from the dispersion has a poor mechanical strength; and
the water glass has an insufficient dispersibility due to its low anion activity.
Meanwhile, the construction and painting industries suffer from pollution-related problems, such as contamination to the building exteriors and to other paint films on building structures. The pollutants include combustion products such as carbon blacks, and inorganic substances such as urban dust and clay particles. This diversity of the pollutants complicates the antifouling measures. It was widely accepted that water-repellent paints, such as polytetrafluoroethylene, would be effective for the prevention of contamination to building exteriors. But it has recently been considered that the film surface should be as hydrophilic as possible against the contamination by urban dust that contains much lipophilic constituents. Thus, proposals have been made to paint buildings with hydrophilic graft polymers. The paint films of graft polymers have a hydrophilicity such that water has a contact angle of 30 to 40°. However, inorganic dusts, typically clay minerals, have a contact angle with water ranging from 20 to 50°. They have an affinity for the graft, polymer paint films having a contact angle with water of 30 to 40° and easily adhere to the film surface. Accordingly, the graft polymer paint films will not be able to prevent the contamination by the inorganic dusts. Many hydrophilic paints are commercially available. They comprise acrylic resins, acryl silicon resins, aqueous silicone, block polymers of silicon resins and acrylic resins, acryl styrene resins, sorbitan fatty acid ethylene oxide, sorbitan fatty acid esters, urethane acetates, urethanes crosslinked with polycarbonate diols and/or polyisocyanates, and crosslinked alkyl polyacrylates. These hydrophilic paints have a relatively large contact angle with water and will not effectively prevent the contamination by city dust that contains much lipophilic constituents.
An alkali silicate aqueous solution called water glass contains relatively large amounts of alkali ions in order to maintain the solution state. The molar ratio of silicon to an alkali (SiO2/(A2O+B) (A: alkali metal, B: NH3)) is usually less than 4. Although the solution contains silicate ions and alkali ions, the negative charge quantity of the solution is small. Therefore, the anionic activity is low and the zeta-potential that is an indication of the anionic activity is in the range of −14 MV or less but more than −40 MV.
On the other hand, primary particles such as silicic acid sol or colloidal silica have neither internal surface area nor crystalline portion, and they are dispersed in an alkaline medium. The alkali reacts with the silica particle to cause a negative charge on the silica surface. The silica particles that have been thus negatively charged stabilize by virtue of the repulsion of the negative charge between the particles. However, many silanol groups (Si—OH) are present on the surface of the silica colloidal substance in addition to the silicate anions that give the negative charge. Consequently, the negative charge quantity is reduced to the zeta-potential of −25 to −38 MV.
Colloidal silica results from the dealkalization of water glass. But a stable intermediate between the water glass and the colloidal silica has not been obtained. The reason is that with the progress of dealkalization, the molar ratio is raised to the extent where the water glass can no longer maintain the solution state. In general, precipitation of silica takes place when the molar ratio is 4.2 or more, and the water glass cannot maintain the solution state.
If a high-molar ratio alkali silicate aqueous solution having solution-like characteristics like the water glass and having a high molar ratio and a high SiO2 concentration similarly to the colloidal silica is used as a component in the photocatalytic coating composition, the composition will produce a dense film having a high film strength and an excellent bond strength. If the film can be given an appropriate hydrophilicity, all the problems described above could be solved at once.
That is to say, it should be studied to use an alkali silicate aqueous solution that has a high molar ratio, a high activity and a high SiO2 concentration without losing solution-like characteristics like the water glass.
However, the molar ratio cannot be raised by simply concentrating the water glass through evaporation. For example, if a water glass product having the highest molar ratio 4.0 is concentrated to an SiO2 concentration of 30% by weight, the product will gel completely.
On the other hand, ultrafiltration is a conventional means for the concentration of colloidal silica (see U.S. Pat. No. 3,969,266, U.K. Patent No. 1,148,950, Japanese Patent Laid-Open Publication No. 15022/1983, etc.). If the silica particles have grown to some extent, the colloidal silica would be satisfactorily concentrated by the ultrafiltation. However, the water glass contains large amounts of low-molecular weight components, such as ions, and therefore the yield obtained by the ultrafiltration is low. Moreover, many ions are lost in the ultrafiltration and the water glass loses its inherent anion activity as a result.
The present invention solves the problems described above, by providing a photocatalytic coating composition that comprises an alkali silicate aqueous solution and a photocatalytic compound, wherein the aqueous solution has intermediate characteristics between water glass and colloidal silica and has a high molar ratio (SiO2/(A2O+B) (A: alkali metal, B: NH3)), a high silicon content and a high anionic activity. This combination enables control of refractive index, film density and hydrophilicity and gives a film with improved strength and adhesion.